The school district said the image dated from last spring and was not taken on school grounds.

It is believed to have been snapped on the steps of a local Sauk County Courthouse in Baraboo, about 120 miles (190km) west of Milwaukee.

The picture was originally posted to a local photographer's website with other photos from the junior prom, but has since been deleted.

Baraboo School district superintendent Lori Mueller wrote in a letter sent to parents Monday: "If the gesture is what it appears to be, the district will pursue any and all available and appropriate actions, including legal, to address the issue."